In recent years, along with the developments of various small-size, lightweight electronic apparatuses, the packaging density of electronic parts becomes higher, and there have been various demands for characteristics required for various electronic parts and materials to be used in these apparatuses. In particular, with respect to printed circuit boards, the circuit occupied area becomes smaller, and comes to have a higher density, and there have been increasing demands for multi-layer circuit boards (built-up circuit boards), flexible wiring boards (FPC) and the like. With respect to these circuit boards, various adhesives and adhesive films are used in manufacturing processes thereof, and with respect to resins to be used for the adhesives, epoxy resins, acrylic resins and the like are mainly used. However, none of these resins sufficiently satisfy characteristics such as heat resistance and electrical insulating property.
Here, adhesives made from polyimide resins and polyamideimide resins have been known as adhesives having superior heat resistance and electrical insulating property; however, these adhesives have a problem in that a thermal stress is generated between a coated member and the adhesive due to thermal history during circuit board manufacturing processes to cause warping in the resulting wiring circuit board.
Conventionally, with respect to the non-flammable agent for materials for various printed circuit boards, halogen-based compounds and antimony-based compounds, such as brome-based compounds, having superior non-flammable effects have been generally used. However, with respect to the halogen-based compounds, recent researches have implied the possibility of generating gases containing dioxin and the like highly toxic to the human body, upon burning, and the application thereof has been limited centered on European countries. With respect to substitutes for such non-flammable agents, well-known specific examples include inorganic fillers, such as aluminum hydroxide and magnesium hydroxide, and non-flammable agents such as phosphor-based compounds.
However, with respect to these compounds, in order to obtain a sufficient non-flammable property, a great amount thereof needs to be added to a target resin, with the result that the inherent characteristics of the resin tend to deteriorate greatly. More specifically, it has been known that, for example, in an FPC-use adhesive among various circuit-board-use adhesives, due to soluble sodium mixed therein upon production, aluminum hydroxide tends to cause a hydrolytic reaction on the surface of a polyimide film that is a coated member, when subjected to a long-time high-temperature, high-humidity process, to weaken the surface of the polyimide film to cause degradation in the peeling strength. Moreover, it has been generally known that magnesium hydroxide causes degradation in acid resistance. Furthermore, phosphates, which have been well known materials among phosphor-based compounds, function as a plasticizer to cause degradation in the heat resistance and the like; therefore, it is necessary to limit the kinds and added amounts thereof.